1. Field of the Invention
This invention relates to a process for removing pollutants from gas streams; more particularly, it relates to a process for removing sulfur oxides from waste gas streams containing same. In a preferred embodiment, this invention relates to an improved double alkali process for removing sulfur dioxide from waste gas streams containing same.
2. Description of the Prior Art
For various reasons, it is necessary and desirable to remove pollutants such as sulfur oxides, particularly sulfur dioxide (SO.sub.2), from gas streams containing same. A prime source of such gas streams comprises the waste gases derived from the combustion of sulfur-containing fuels, although they can also be generated by other gas producing processes.
Many different processes have been suggested for removing sulfur oxides such as sulfur dioxide from waste gases including, for example, scrubbing with a solution of sodium carbonate or sodium hydroxide; scrubbing with lime or a limestone slurry; or by the so-called double alkali process. The first process is simple but its application is limited since the resulting dissolved solids must be treated and fed to waste in a liquid stream. The lime or limestone scrubbing process produces a low-solubility solid waste, but although the cost of scrubbing chemicals is low, the capital requirements are high due to the high circulation rates required and the possibility of deposition of solid scale in various parts of the system which necessitates periodic cleaning and/or replacement.
The double alkali process remedies some of these deficiencies and generally comprises scrubbing the sulfur oxide-containing gas with an alkali such as a soluble sodium or potassium solution, followed by the addition of another alkali such as lime to at least a portion of the resulting scrubber effluent to regenerate the alkali scrubbing solution. The primary advantage of the double alkali process is that the scrubbing step produces only soluble materials and that insoluble materials are formed only in areas outside the scrubber.
A double alkali process is disclosed in U.S. Pat. No. 3,911,084. As disclosed therein, a circulating aqueous extracting/scrubbing solution of sodium sulfite and sodium bisulfite is provided having a specified pH of 6-7. Make-up for the extracting solution may be obtained using a soluble alkali sodium compound solution with a pH of 8-12. Upon contact with the sulfur dioxide-containing gas stream, SO.sub.2 is absorbed by the extracting/scrubbing solution and an extract is formed having a relatively lower pH and a relatively higher sodium bisulfite content. At least a portion of the extract is regenerated in a single step by reaction with a calcium compound such as lime to form a slurry containing dissolved sodium sulfite and solid calcium sulfite and having a pH of 8-10. After separating solids therefrom, the regenerated sodium sulfite solution is mixed with the circulating extracting/scrubbing solution to maintain is pH at 6-7. However, such double alkali processes were deficient in that the regeneration efficiency was not high and a predominantly sulfite filter cake was produced which is difficult to handle.
Commonly-assigned U.S. Pat. No. 4,313,924 remedies such deficiencies with an improved multi-stage regeneration procedure using lime to regenerate sulfite from bisulfite. The bleed from the scrubbing step which is subjected to regeneration has a low pH and hence can dissolve more lime thereby increasing the lime utilization efficiency. In the regeneration procedure, a lime slurry is separated into a slurry of coarser lime particles and a second portion which contains very fine lime particles. The main regeneration is accomplished by adding the coarse lime slurry to the scrubber bleed. Due to the low pH in this reaction, not all the bisulfite is regenerated to sulfite. However, any remaining bisulfite is regenerated to sulfite in later regeneration steps by reaction with the second portion of the lime slurry in order to precipitate as much calcium as possible in the form of sulfate.
Other processes for removing sulfur dioxide from waste gases are disclosed in, for example, U.S. Pat. Nos. 3,775,532; 3,653,812; 3,477,815; 3,485,581; 3,989,797; 3,944,649; 4,021,202; 3,542,511; 3,607,033; 3,622,443; 1,271,899; and Japanese Patent No. 49-48599.
Most prior art sodium-based double alkali processes utilize caustic (i.e., NaOH) or soda ash (i.e., sodium carbonate, Na.sub.2 CO.sub.3) as make-up to the scrubbing solution. Such a scrubbing solution always contains sodium bisulfite (NaHSO.sub.3) and sodium sulfite (Na.sub.2 SO.sub.3) since the reactions shown in the following equations (1) and (2) occur spontaneously and reaction (3) represents the main SO.sub.2 absorption reaction: EQU Na.sub.2 CO.sub.3 +NaHSO.sub.3 .fwdarw.2Na.sub.2 SO.sub.3 +CO.sub.2 +H.sub.2 O (1) EQU NaOH+NaHSO.sub.3 .fwdarw.Na.sub.2 SO.sub.3 +H.sub.2 O (2) EQU SO.sub.2 +Na.sub.2 SO.sub.3 +H.sub.2 O.fwdarw.2NaHSO.sub.3 ( 3)
In addition, the scrubbing solution may also contain sodium sulfate (Na.sub.2 SO.sub.4) due to oxidation of sodium sulfite by any free oxygen in the gases and also because of the possible formation of sodium sulfate during regeneration which is recycled to the scrubbing step. Therefore, as a result of the scrubbing or absorption step, an aqueous solution is produced containing relatively less sodium sulfite, relatively more sodium bisulfite and some sodium sulfate.
In one possible manner of regeneration, a calcium compound such as lime, Ca(OH).sub.2, is added and reacts with the sodium bisulfite forming solid calcium sulfite (CaSO.sub.3) and regenerating sodium sulfite as shown in equation (4): EQU 2NaHSO.sub.3 +Ca(OH).sub.2 .fwdarw.CaSO.sub.3 +Na.sub.2 SO.sub.3 +H.sub.2 O (4)
However, depending on the pH and other conditions of the regenerating reaction, one or both of solid calcium sulfate (CaSO.sub.4) and dissolved sodium sulfate may be formed. Specifically, at a pH of less than 8 and higher than 10, a greater portion of solid calcium sulfate tends to form rather than solid calcium sulfite but since the former is more water soluble than the latter and hence tends to go back into solution as sodium sulfate, while more calcium sulfite precipitates, a solid phase equilibrium is reached. Hence, depending predominantly on the pH of the regeneration, some sodium sulfate may be formed in the thickener and recycled to the scrubbing step with the regenerated sodium sulfite. In fact, the sodium sulfate concentration is permitted to equilibrate in the process disclosed in the aforementioned U.S. Pat. No. 3,911,084, in order to reduce the undesired removal of sulfite/bisulfite values with the solid precipitate.
Most prior art double alkali systems require a "softening" step to remove or reduce the concentration of dissolved calcium ions to prevent precipitation of calcium salts in the scrubber which foul the system. This is usually done in a clarifier by adding soda ash, Na.sub.2 CO.sub.3, a relatively expensive form of active sodium, wherein the carbonate ions react with the dissolved calcium ions to precipitate calcium in the form of calcium carbonate, CaCO.sub.3, which is extremely insoluble in water. Since other losses of sodium are usually minimal, the amount of soda ash used in the softening step often controls the amount required for the process. Hence, most prior art double alkali processes often do only a minimum amount of softening in view of the expense of soda ash and therefore frequently experience scale problems due to precipitated calcium salts.
U.S. Pat. No. 4,231,995 discloses an ammonia double alkali process which includes scrubbing a gas stream containing sulfur oxide with an ammonium sulfite solution; and regenerating at least a portion of the ammonium compound by treating the resulting scrubber effluent with a source of calcium ions, wherein the precipitation of gypsum is discouraged to control the calcium concentration of the process liquor.
U.S. Pat. No. 3,870,781 discloses a process for treating sulfur oxide - containing gases which comprises scrubbing with an aqueous alkali metal hydroxide solution (e.g., NaOH) containing an alkaline earth metal sulfate (e.g., CaSO.sub.4), converting (oxidizing) the resulting bisulfite (NaHSO.sub.3) to the corresponding sulfate (Na.sub.2 SO.sub.4) and hydrogen ions (H.sub.2 SO.sub.4) and reacting these ions with an alkaline earth metal compound (e.g., Ca(OH).sub.2) to precipitate an alkaline earth metal sulfate (CaSO.sub.4) and thereby reform the alkali metal hydroxide scrubbing solution. At column 3, lines 22-31, the patentees indicate that all sulfites are removed from the system to avoid scaling and plugging problems and that the air oxidation of the scrubber effluent eliminates CO.sub.2 from the system thus preventing loss of alkali due to CaCO.sub.3 precipitation.
U.S. Pat. No. 3,873,532 discloses a sodium based scrubbing process for removing sulfur dioxide from gases containing same and includes the following steps:
(1) scrubbing the gas with an aqueous alkali solution such as sodium hydroxide or sodium carbonate to produce an aqueous stream containing the corresponding sulfite and sulfate ions;
(2) splitting the scrubber effluent into two streams and recycling the major stream to the scrubbing step;
(3) reacting the minor stream with a calcium compound such as slaked lime to regenerate sodium hydroxide and precipitate out calcium as sulfite and sulfate;
(4) conveying the resulting suspension to a settling device and feeding the settler underflow to a filter wherein a minor portion of the underflow is recycled to the lime regeneration reaction, wherein the presence therein of the calcium sulfate crystals enhances the removal of calcium sulfite and sulfate from the reacted liquid; and
(5) preferably, the overflow from the settling device is fed to a clarifier to which is also added sodium carbonate to reduce the concentration of dissolved calcium ions in the clarified regenerated scrubbing liquid, which is recycled back to the lime regeneration reaction. In one embodiment, a fraction of the exhaust gases from the scrubbing step is injected into the settler overflow fed to the clarifier wherein the carbon dioxide in the exhaust gases reacts with the calcium ions dissolved in that liquid stream to precipitate calcium carbonate and partially convert sodium hydroxide to sodium carbonate, the latter which then reacts with the calcium sulfate/sulfite in the system to reduce the amount of sodium carbonate required. It has been found however that the process described in this patent results in overcarbonization of the liquid in the clarifier. Specifically, it has been observed that sodium bicarbonate, NaHCO.sub.3, is formed therein which in turn forms calcium bicarbonate, CaHCO.sub.3, which is at least partially soluble in the clarifier. To the extent these bicarbonates are formed in the clarifier, there is no carbonate precipitate, instead a relatively gelatinous mass is formed which is difficult to filter. In addition, some make-up sodium carbonate is required.